Why is it that the gods used flooding to kill humans? Why not fire or a plague?
From what I understand of the "Claims of historicity" section in the article, there were many great floods in the wake of the last glacial period which would naturally be associated the wrath of the gods.
Plague and large fires would be rarer in the past, with epidemics not occurring very often until larger communities evolved
There are several changes that may occur in an infectious agent that may trigger an epidemic. These include:
- Increased virulence
- Introduction into a novel setting
- Changes in host susceptibility to the infectious agent
with bad waste management causing pollution, leading to this:
The conditions which govern the outbreak of epidemics include infected food supplies such as contaminated drinking water and the migration of populations of certain animals, such as rats or mosquitoes, which can act as disease vectors. Certain epidemics occur at certain seasons.
(both citations from Epidemic, Wikipedia)
Fires rarely used to spread very far due to geographical features and regular wildfires, as talked about in this TEDx talk, making them less threatening than floods.
I hope this answers your question at least from a scientific point of view.